Chapter 12: General Sensation

Learning Objectives

  • L.O. 12.1 Outline an overview of sensation and perception.

  • L.O. 12.2 Summarize each classification of sensory receptors, including by location, modality, structure and differences in sensory adaptation.

  • L.O. 12.3 Explain an overview of key sensory receptors and pathways.

  • L.O. 12.4 Describe sensory receptor physiology, including how signals are converted from analog to digital.

  • L.O. 12.5 Summarize the two somatosensory pathways.

  • L.O. 12.6 Describe what happens during aging of sensory receptors.

Overview of Sensation and Perception

  • Sensation: The process of detecting stimuli from the environment.

    • General somatic sensations include:

    • Touch

    • Pressure

    • Temperature

    • Proprioception (awareness of body position)

    • Nociception (pain)

    • Visceral sensations relate to internal organ stimuli.

  • Perception: The process of interpreting sensory information; assigning meaning to sensory inputs.

Classification of Sensory Receptors

By Location

  • Exteroreceptors

    • Function: Respond to stimuli from the external environment.

    • Location: Situated at or near body surfaces.

    • Example: Receptors in the skin.

  • Interoreceptors

    • Function: Respond to internal changes in the body.

    • Types:

    • Visceral interoreceptors detect changes within internal organs.

    • Somatic interoreceptors located in musculoskeletal structures, provide information about the body's interaction with the environment.

By Modality

  • Thermoreceptors

    • Function: Sense changes in temperature.

  • Chemoreceptors

    • Function: Sense changes in chemical stimuli (e.g., blood gases and pH levels).

  • Mechanoreceptors

    • Function: Sense mechanical forces and changes in pressure, gravity, changes in cell volume/shape, position, touch, itch, and movement.

    • Examples: Touch receptors, baroreceptors, osmoreceptors, proprioceptors.

  • Nociceptors

    • Function: Sense tissue damage, physical trauma, or thermal injury.

By Structure

  • Unipolar neuron with dendrites as sensory receptors

  • Unipolar neuron with encapsulated dendrites

  • Sensory cell synapsing directly onto a neuron

  • Neuron with peripheral processes

Sensory Adaptation

  • Definition: Sensory adaptation is the loss of responsiveness at the sensory receptor level when exposed to a constant stimulus.

Mechanoreceptors and Receptive Fields

  • Sensory Unit: The sensation generated by a sensory neuron and all its receptors.

  • Receptive Field: The area of surface covered by a single sensory unit.

    • A larger sensory field complicates stimulus localization.

Types of Cutaneous Mechanoreceptors
Free Nerve Endings

  • Anatomy: Unencapsulated, penetrating dermis, ending in stratum granulosum.

  • Features: Detect temperature, touch, pressure, stretch, and cell damage; adapt slowly to rapidly.

Merkel Cells

  • Anatomy: Oval-shaped mechanoreceptors located in the skin's stratum basale.

  • Features: Highly sensitive to fine touch and texture; provide steady-state signals; slowly adapting.

Meissner's Corpuscles

  • Anatomy: Encapsulated nerve endings with horizontal lamellae.

  • Features: Sensitive to light touch and vibrations (10–50 Hz); concentrated in fingertips and lips.

Ruffini Corpuscles

  • Anatomy: Free nerve endings intertwined with collagen fibers; encapsulated.

  • Features: Sensitive to skin stretch; critical for grasping objects; respond to sustained pressure with little adaptation.

Krause’s Corpuscles

  • Anatomy: Small, round, or oval-shaped encapsulated mechanoreceptors.

  • Features: Found in dermis of dry skin and mucous membranes; detect pressure and cold temperatures.

Pacinian Corpuscles

  • Anatomy: Nerve endings within concentric layers of collagen fibers; enclosed by a capsule.

  • Features: Located in deep part of dermis and fascia, sensitive to deep pressure and vibration; very rapidly adapting.

Thermoreceptors

  • Detect sensations of cold and heat.

  • Transient Receptor Potential (TRP) Channels: Specialized proteins that open at different temperatures, releasing Ca2+ to help detect temperature changes in humans.

Nociceptors

General Concepts

  • Characteristics: Concentrated in areas prone to injury; absent from the brain and sparse in most internal organs; adapt very slowly or not at all.

  • Types of Pain:

    1. Fast pain

    2. Slow pain

    3. Acute pain

    4. Chronic pain

    5. Visceral pain (which can be referred).

Pain Sensory Pathways

  • Signals travel through:

    • First-order neuron: From sensory receptors to the spinal cord.

    • Second-order neuron: In the spinal cord up to the thalamus.

    • Third-order neuron: From the thalamus to the somatosensory cortex.

Pharmacological Modifiers of Pain

  • Analgesics: Modify the perception of pain (e.g., NSAIDs).

  • Anesthetics: Block action potential generation in primary afferent fibers (e.g., lidocaine or tetracaine).

  • Endogenous Opioids: Chemicals produced by the body that modulate pain (e.g., β-endorphins).

  • Synthetic Opioids: Inhibit pain transmission at central synapses (e.g., morphine).

  • Anticonvulsants: Effective for symptoms of neuropathic pain (e.g., carbamazepine).

Sensory Receptor Physiology: Analog to Digital

The Relationship Between Stimulus Strength and Action Potentials

  • Stimulus: Strength affects the receptor membrane potential.

  • Graded Potentials: Analog signals that change in magnitude according to stimulus intensity.

  • Action Potentials: Digital signals sent down the axon when threshold is reached; generated at the first node of Ranvier.

Recruitment and Range Fractionation

  • Recruitment: Stronger stimuli activate more sensory units; the intensity of sensory information is based on:

    • Increased action potential frequency per sensory unit.

    • Increased number of sensory units activated.

  • Range Fractionation: Subsets of receptors respond to distinct ranges of stimuli.

Lateral Inhibition

  • Increases the sensitivity of sensory cells; primary neurons respond proportionally to stimulus strength while secondary neurons inhibit neighbors, enhancing stimulus perception.

Somatosensory Pathways

Dorsal Column Pathway

  • Anatomy & Features: This major pathway detects fine touch and proprioception.

    • Sensory Neurons: Cell body in dorsal root ganglion, project into dorsal column, synapsing with interneurons in the medulla.

    • Interneurons: Ascend as medial lemniscus to synapse with third-order neurons in the thalamus.

    • Third-order Neurons: Project axons to the somatosensory cortex.

Anterolateral Pathway

  • Anatomy & Features: Conveys pain and temperature information.

    • Sensory Neurons: Begin in dorsal root ganglion; synapse with interneurons in dorsal horn.

    • Interneurons: Decussate from spinal cord to thalamus.

    • Third-order Neurons: Project axons to the somatosensory cortex.

Aging of Sensory Receptors

Features of Aging Sensation

  • Declining Sensitivity: Sensory structures (e.g., vibration, touch, proprioception) show decreasing sensitivity.

  • Increased Threshold: Higher stimulation required for action potential generation.

  • Decreased Chemoreceptor Sensitivity: Reduced response to hypoxia and other chemical changes.

  • Lower Sensitivity to Pain: Pain thresholds may increase with age, impacting pain perception.